Electrical System of the Heart

Topic Overview

What controls the timing of your heartbeat?

Your heart's electrical system controls the timing of your heartbeat by regulating your:

• Heart rate, which is the number of times your heart beats per minute.
• Heart rhythm, which is the synchronized pumping action of your four heart chambers.

Your heart's electrical system should maintain:

• A steady heart rate of 60 to 100 beats per minute at rest. The heart's electrical system also increases this rate to meet your body's needs during physical activity and lowers it during sleep.
• An orderly contraction of your atria and ventricles (this is called a sinus rhythm).

See a picture of the heart and its electrical system.

How does the heart's electrical system work?

Your heart muscle is made of tiny cells. Your heart's electrical system controls the timing of your heartbeat by sending an electrical signal through these cells.

Two different types of cells in your heart enable the electrical signal to control your heartbeat:

• Conducting cells carry your heart's electrical signal.
• Muscle cells enable your heart's chambers to contract, an action triggered by your heart's electrical signal.

The electrical signal travels through the network of conducting cell "pathways," which stimulates your upper chambers (atria) and lower chambers (ventricles) to contract. The signal is able to travel along these pathways by means of a complex reaction that allows each cell to activate one next to it, stimulating it to "pass along" the electrical signal in an orderly manner. As cell after cell rapidly transmits the electrical charge, the entire heart contracts in one coordinated motion, creating a heartbeat.

The electrical signal starts in a group of cells at the top of your heart called the sinoatrial (SA) node. The signal then travels down through your heart, triggering first your two atria and then your two ventricles. In a healthy heart, the signal travels very quickly through the heart, allowing the chambers to contract in a smooth, orderly fashion.

The heartbeat happens as follows:

1. The SA node (called the pacemaker of the heart) sends out an electrical impulse.
2. The upper heart chambers (atria) contract.
3. The AV node sends an impulse into the ventricles.
4. The lower heart chambers (ventricles) contract or pump.
5. The SA node sends another signal to the atria to contract, which starts the cycle over again.

This cycle of an electrical signal followed by a contraction is one heartbeat.

SA node and atria

When the SA node sends an electrical impulse, it triggers the following process:

• The electrical signal travels from your SA node through muscle cells in your right and left atria.
• The signal triggers the muscle cells that make your atria contract.
• The atria contract, pumping blood into your left and right ventricles.

AV node and ventricles

After the electrical signal has caused your atria to contract and pump blood into your ventricles, the electrical signal arrives at a group of cells at the bottom of the right atrium called the atrioventricular node, or AV node. The AV node briefly slows down the electrical signal, giving the ventricles time to receive the blood from the atria. The electrical signal then moves on to trigger your ventricles.

When the electrical signal leaves the AV node, it triggers the following process:

• The signal travels down a bundle of conduction cells called the bundle of His, which divides the signal into two branches: one branch goes to the left ventricle, another to the right ventricle.
• These two main branches divide further into a system of conducting fibers that spreads the signal through your left and right ventricles, causing the ventricles to contract.
• When the ventricles contract, your right ventricle pumps blood to your lungs and the left ventricle pumps blood to the rest of your body.

After your atria and ventricles contract, each part of the system electrically resets itself.

How does the heart's electrical system regulate your heart rate?

The cells of the SA node at the top of the heart are known as the pacemaker of the heart because the rate at which these cells send out electrical signals determines the rate at which the entire heart beats (heart rate).

The normal heart rate at rest ranges between 60 and 100 beats per minute. Your heart rate can adjust higher or lower to meet your body's needs.

What makes your heart rate speed up or slow down?

Your brain and other parts of your body send signals to stimulate your heart to beat either at a faster or a slower rate. Although the way all of the chemical signals interact to affect your heart rate is complex, the net result is that these signals tell the SA node to fire charges at either a faster or slower pace, resulting in a faster or a slower heart rate.

For example, during periods of exercise, when the body requires more oxygen to function, signals from your body cause your heart rate to increase significantly to deliver more blood (and therefore more oxygen) to the body. Your heart rate can increase beyond 100 beats per minute to meet your body's increased needs during physical exertion.

Similarly, during periods of rest or sleep, when the body needs less oxygen, the heart rate decreases. Some athletes actually may have normal heart rates well below 60 because their hearts are very efficient and don't need to beat as fast. Changes in your heart rate, therefore, are a normal part of your heart's effort to meet the needs of your body.

How does your body control your heart rate?

Your body controls your heart by:

• The sympathetic and parasympathetic nervous systems, which have nerve endings in the heart.
• Hormones, such as epinephrine and norepinephrine (catecholamines), which circulate in the bloodstream.

Sympathetic and parasympathetic nervous systems

The sympathetic and parasympathetic nervous systems are opposing forces that affect your heart rate. Both systems are made up of very tiny nerves that travel from the brain or spinal cord to your heart. The sympathetic nervous system is triggered during stress or a need for increased cardiac output and sends signals to your heart to increase its rate. The parasympathetic system is active during periods of rest and sends signals to your heart to decrease its rate.

Catecholamines

During stress or a need for increased cardiac output, the adrenal glands release a hormone called norepinephrine into the bloodstream at the same time that the sympathetic nervous system is also triggered to increase your heart rate. This hormone causes the heart to beat faster, and unlike the sympathetic nervous system that sends an instantaneous and short-lived signal, norepinephrine released into the bloodstream increases the heart rate for several minutes or more.